Polyoxymethylene is well known for its propensity to physically degrade when exposed to ultraviolet (UV) light for long periods of time. This degradative effect of UV light on polyoxymethylene impairs its toughness and flexibility as well as promoting discoloration. Toughness and color retention are necessary criteria when polyoxymethylene compositions are employed in automotive applications (e.g., automotive interior components).
In addition to the requirements of toughness and color retention, any successful polyoxymethylene composition must also exhibit desirable processability characteristics. That is, during molding of parts, the composition must be capable of releasing easily from the mold without forming significant amounts of mold deposits. Otherwise, the molded part itself may not be useable due to improper and/or inconsistent part surfaces or unacceptably high surface gloss levels (particularly where parts having low surface gloss levels are desired).
It is known from U.S. Pat. No. 3,219,621 to Prichard et al issued Nov. 23, 1965 (the entire content of which is incorporated expressly hereinto by reference) that the susceptibility of oxymethylene polymers to "weathering" (including discoloration due to exposure to UV light) without undue decrease in thermal stability can be reduced by the addition of 2-hydroxy benzophenone to a polyoxymethylene polymer wherein the polymer units are derived from cyclic ethers having at least two adjacent carbon atoms.
It is also known that molding compositions of oxymethylene copolymers (i.e., having at least one chain containing from about 85 to 99.6 mole percent of oxymethylene units interspersed with about 0.4 to 15 mole percent of oxyethylene units) can be rendered less susceptible to the degradative effects of UV light by incorporating in the composition between 0.1 to 1.0 percent by weight of 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4 hydroxy hydrocinnamate, about 0.015 to 0.4 percent by weight of 2,4,6-triamino-sym-triazine, about 0.1 to 0.75 percent by weight of bis(l,2,2,6,6-pentamethyl-4-piperidinyl) (3,5-bis(1,1-dimethylethyl-4-hydroxyphenol)methyl))butyl propanedioate and about 0.25 to 1.0 percent by weight of 2-hydroxy-4-n-octoxybenzophenone. In this regard, see U.S. Pat. No. 4,446,263 issued to Walter R. Bryant on May 1, 1984, the entire content of which is expressly incorporated hereinto by reference.
While the above-noted polyoxymethylene formulations do provide enhancement against the degradative effects of UV light, additional improvements in this regard are still needed. Thus, it is towards achieving improvements to UV-light stabilized polyoxymethylene compositions that the present invention is directed.